1. Field of the Invention
This invention relates to the recovery of heavy oils and tars from steeply dipping reservoirs penetrated by a plurality of wells and more particularly to steam flooding operations involving the same.
2. Description of the Prior Art
Petroleum reservoirs are found in an almost incredible variety. Those of interest herein are steeply dipping reservoirs, which may or may not outcrop at the surface, and which contain predominantly high viscosity petroleum fractions such as heavy oils and tars.
In a dipping reservoir, one of the most commonly used production techniques is gravity drainage wherein production wells are drilled in the lower portion of the reservoir allowing the oil to flow downdip under the influence of gravity to the producing wells wherefrom the petroleum is either flowed or pumped to the surface. The rate of the downward oil flow is known to be proportional to a term: ##EQU1## where .kappa..sub.o is the oil permeability, .mu..sub.o is the oil viscosity, .rho..sub.o is the oil density, .rho..sub.g is the gas density and .alpha. is the reservoir dip angle.
For reservoirs exhibiting strong gravity drainage characteristics, the value of the above term ranges from about 10 to 200 where .kappa..sub.o is expressed in millidarcies, .mu..sub.o in centipoise, .rho..sub.o and .rho..sub.g in grams per cubic centimeter. It is immediately evident that for a steeply dipping reservoir containing highly viscous petroleum the value of the above term at the initial reservoir temperature will be much less than 10 due to the high value of the oil viscosity. Basic reservoir engineering knowledge indicates that the most effective means to reduce the oil viscosity in such a situation is to inject steam or hot water into the reservoir. The heat from the injected fluids serves to raise the temperature of the reservoir with a resultant reduction in the viscosity of the petroleum contained therein. Consequently, the value of the above term can be increased to within the desirable range of about 10 to 200 thereby creating a favorable gravity drainage condition for the dipping reservoir containing the high viscosity petroleum. Other beneficial effects concommitant with steam injection are thermal expansion of the petroleum, beneficial wetting of reservoir porous media surfaces by the condensed heated water and increased overall pressure within the reservoir.
Nevertheless, the injection of steam into a dipping reservoir presents a number of problems. The natural tendency of the injected steam, due to its low density and high mobility, is to flow upwards in the formation to the updip limit of the reservoir. This updip limit is commonly encountered in the form of a permeability barrier formed by the petroleum itself and/or outcrop of the reservoir at the surface. The injected steam can, however, easily break through this permeability barrier with a extremely detrimental results. If the updip permeability barrier is found in the subsurface, the steam and mobilized hydrocarbons will escape into adjacent formations and be lost. If the updip permeability barrier is found at the surface, steam breakthrough will result in an escape of the gaseous fluid to the atmosphere creating serious environmental pollution problems as well as severely damaging the future production potential for the reservoir.
Heretofore avoidance of steam breakthrough at the updip permeability barrier has been achieved only by those methods which employ extremely conservative steam injection rates and the shutting-off of steam injection wells which were felt to be in too close proximity to the permeability barrier. Such production practices, while prudent, will often leave substantial areas of the reservoir essentially untapped by the steam injection program due to the low injection rates and avoidance of steam injection into the upper portions of the reservoir for fear of a steam breakthrough. There remains an unmet need to utilize the full potential of an efficient steam injection program involving the maximum possible volume of the reservoir and optimal steam injection rates in a dipping heavy oil reservoir while concurrently avoiding the problem of steam breakthrough at the updip permeability barrier.